Answer:
2733.73
Step-by-step explanation:
[tex]r = \frac{kl}{ {d}^{2} } [/tex]
[tex]17113 = \frac{300k}{ {0.09}^{2} } [/tex]
[tex]k = 0.462[/tex]
[tex]r = \frac{0.462 \times 900}{ {0.39}^{2} } [/tex]
[tex]r = 2733.73[/tex]
[tex]\qquad \qquad \textit{combined proportional variation} \\\\ \begin{array}{llll} \textit{\underline{y} varies directly with \underline{x}}\\ \textit{and inversely with }\underline{z^5} \end{array} ~\hspace{6em} \stackrel{\textit{constant of variation}}{y=\cfrac{\stackrel{\downarrow }{k}x}{z^5}~\hfill } \\\\[-0.35em] \rule{34em}{0.25pt}[/tex]
[tex]\begin{array}{llll} \textit{\tiny "R" varies directly}\\ \textit{\tiny with "L" and inversely}\\ \textit{\tiny as the square of "d"} \end{array}~~R=\cfrac{kL}{d^2}\qquad \qquad \textit{we also know that} \begin{cases} L=300\\ d=0.09\\ R=17113 \end{cases} \\\\\\ 17113=\cfrac{k(300)}{0.09^2}\implies 17113(0.09^2)=300k \\\\\\ \cfrac{17113(0.09^2)}{300}=k\implies 0.462051 = k~\hfill \boxed{R=\cfrac{0.462051L}{d^2}}[/tex]
[tex]\textit{when L = 900 ft and d = 0.39 in, what is "R"?} \\\\\\ R=\cfrac{0.462051(900)}{0.39^2}\implies R\approx 2734.03[/tex]
